The present invention relates to the fabrication of multi-layer ceramic substrates and, more particularly, relates to the forming of an electrically conductive surface pad that has improved mechanical and electrical reliability and is suitable for high frequency applications.
Multilayer ceramic substrates are typically used for inter-connection between integrated circuit devices used in information processing systems such as computers, control systems and circuit boards. A method to make multilayer ceramic substrates involves forming the dielectric material into a tape by using a casting process, punching (or perforating) the tape to form holes or vias into which a conductive metal paste is deposited along with metal traces that act as wiring. A number of these punched and metallized tapes are aligned, stacked and pressed into a laminate that is subsequently sintered. The sintering is by a cofired process in which the ceramic and metal powders are consolidated in essentially a single but often complex heating process.
A key element of the sintering process involves the formation on the ceramic of conductive surface features (e.g., input/output (I/O) pads) onto which metal plating such as nickel and gold may be deposited, sometimes as preparation for subsequent solder attach processes. These surface features provide connections to the electrical conductors within the ceramic and their mechanical and electrical performance are very important to the reliability of the multilayer ceramic substrate and the entire information processing system.
The strength of the bond between the surface features and the ceramic is important when the surface features, typically present as an array, are electrically connected to a printed circuit board by a solder technique such as ceramic ball grid array (CBGA) or ceramic column grid array (CCGA).
High metal content surface features are desirable because of improved electrical conductivity as well as better plateability of the surface features. However, there is often a weak bond between high metal content surface features and the underlying ceramic. It has been found by Fasano et al. U.S. Pat. No. 6,312,791, the disclosure of which is incorporated by reference herein, that the reliability of high content metal features can be improved by anchoring them to an inner pad which is well bonded to the ceramic material. As taught by Fasano et al. and as illustrated in FIG. 1 herein, anchoring of the metal feature 10 may occur by a plurality of vias 12, some of which 14 may be electrically nonfunctioning. In one embodiment taught by Fasano et al. and as shown in FIG. 2 herein, the surface feature 10 is held in place by one large via 16 bonded to an inner pad 18 which, in turn, is held in place by a plurality of vias 20, again some of which 22 may be electrically nonfunctioning.
The present inventors have found that the structure disclosed in Fasano et al. is not preferable for high frequency applications in the gigahertz frequency range. To meet electrical requirements for high frequency applications, it is necessary to have a structure with only single vias. However, such single via structures present mechanical reliability challenges as noted in Fasano et al.
Various other solutions have been proposed for anchoring a bond pad or other feature to a substrate. Yu et al. U.S. Pat. No. 6,501,186 discloses a bond pad which is anchored by a plurality of vias which improve the mechanical strength of the bond pad. Fork et al. U.S. Pat. No. 6,361,331 discloses a metal strap which is used to anchor a spring metal finger and pad to a substrate. Hashemi U.S. Pat. No. 6,252,178 discloses a semiconductor device with bonding anchors in build-up layers by using non-stacked vias in the build-up layers. Lin et al. U.S. Pat. No. 6,181,016 discloses a bond pad with a single anchoring structure which comprises a plurality of interconnected line segments. Schreiber et al. U.S. Pat. No. 5,738,530 discloses a contact pad having an elastomeric electrical contact anchored by an electrically conductive metallic trace. Barrow U.S. Pat. No. 5,706,178 discloses an elliptically shaped solder pad which is partially covered by a so-called solder mask so that the solder pads are anchored by the solder mask. Ichkhan U.S. Pat. No. 4,900,878 discloses plated-through holes which are anchored by anchor pads around the plated-through holes. All of the foregoing references are incorporated by reference herein.
Notwithstanding all of the prior efforts of those skilled in the art, there still remains a need for a mechanically robust surface feature which is acceptable for high frequency applications in the gigahertz frequency range.
Accordingly, it is a purpose of the present invention to have a mechanically robust surface feature.
It is another purpose of the present invention to have an anchored surface feature which is acceptable for high frequency applications in the gigahertz frequency range and is also mechanically robust.
These and other purposes will become more apparent after referring to the following description of the invention considered in conjunction with the accompanying drawings.